1. Field of the Invention
The present invention relates to apparatus for stretching an optical fiber, particularly to alter the optical characteristics of a Bragg grating incorporated in the fiber. The stretching may be used to determine whether a light of certain wavelengths passes through the fiber or is reflected by the grating. The grating can be tuned to particular wavelengths by stretching the fiber.
2. Prior Art
It is known that stretching an optical fiber can alter certain optical characteristics, especially where the optical fiber incorporates a Bragg grating. Such a grating comprises a series of periodic variations in the refractive index of the core of the fiber, and is effective to reflect certain wavelengths of light back along the fiber while allowing other wavelengths to pass.
It has been proposed to alter the reflective characteristics of Bragg grating by stretching or longitudinally compressing the fiber portion containing the grating. Various methods for stretching are discussed in the following U.S. patents: U.S. Pat. No. 5,007,705 issued Apr. 16, 1991 to Morey et al.; and U.S. Pat. No. 5,469,520, issued Nov. 21, 1995 also to Morey et al.
Methods of stretching suggested in these prior patents include:
a) bonding the fiber sections on opposite sides of the grating to locations on a support made of material having a larger coefficient of expansion than that of the fiber, and then varying the temperature of the support;
b) using an electromechanical device, specifically a piezoelectric device, or a stack of such devices, for stretching the fiber, or wrapping the fiber around a cylinder of piezoelectric material to which a varying voltage is applied; and
c) coating the fiber with a metal, for example nickel, which is sensitive to electric or magnetic or acoustic fields, and exposing the fiber to such fields which are varied to stretch the fiber.
These methods have the drawback that they may not give a precise amount of stretch to the fiber and thus do not reliably alter the particular wavelengths of light passed or reflected by the grating. It is considered desirable to have a device which changes the length of a portion of fiber between precise limits, which limits will preferably be adjustable. Such a device can be used to switch the grating between two conditions. For example, in a bistable device, the light which is back reflected from a Bragg grating can be switched between two wavelengths, which can determine whether data is allowed to follow an express throughput path or whether it is back-reflected and re-routed.
Another drawback of some of the prior art devices is that in operation some of these have significant hysteresis losses, and therefore energy losses.
In accordance with the present invention, apparatus for modifying the optical characteristics of an optical fiber by controlled stretching of a length of the fiber, comprises:
a base;
first fiber holding means fixedly carried by the base and capable of firmly holding a first portion of the optical fiber;
a lever part attached to the base at a fulcrum;
second fiber holding means connected to the lever part and movable therewith, and capable of firmly holding a second portion of the fiber such that an intermediate length of the fiber may be held taut between the first and second holding means;
means for moving the lever part between first and second predetermined positions, so that the intermediate length of fiber is stretched as the lever part moves between the first and second positions. The latter means are preferably electromagnetic.
Where the fiber has a Bragg grating, the first and second fiber holding means will of course be located at portions of the fiber on opposite sides of the Bragg grating.
The lever preferably provides a considerable mechanical advantage, so that the movement of the outer end of the lever, which is controlled by adjustable stops, produces a much smaller amount of movement in the second, i.e. the movable fiber holding means. Preferably, the length of the lever part between the fulcrum and the electromagnetic means is at least 10 times the distance between the fulcrum and the second fiber holding means.
The fulcrum may be provided by a flexure which is integral both with the lever part and with a part of said base; this ensures that there is no play at the fulcrum and the movement is substantially frictionless, and energy losses are very small. Also, the lack of bearing surfaces means that this construction has a long life. Preferably, the lever part is releasably held in the first or second positions by magnetic latching means, which positions are determined by adjustable stops.